The Grand Canyon stuns visitors with breathtaking views every day. Between November 29 and December 2, 2013, it stunned visitors even more by not being visible. A rare meteorological event filled the canyon with an ocean of clouds. Such events are so rare that National Park Service rangers—who see the canyon every day—wait for years to see the ground-hugging fog. The photo above from Mather Point is one of more than two dozen that Grand Canyon National Park posted to its Flikr photostream.

The fog was trapped in the canyon by a temperature inversion, which happens when the air near the ground is cooler than the air above it. A high-pressure system brought low temperatures, clear skies, and calm winds to the Grand Canyon. During the long nights, the ground cooled quickly, chilling the air immediately above it. The air higher in the atmosphere did not cool as quickly, and so an inversion developed. Without wind to stir it, cold dense air was trapped beneath a more buoyant layer of warm air.

The inversion was only half of the story. A few days earlier, a winter storm dumped heavy snow and rain on northern Arizona. The National Weather Service reported 11.5 inches (29.2 cm) of snow at the Grand Canyon Rim. The snow and rain left the ground and the air above it very moist. As the air near the ground cooled at night, the water condensed into fog, which was trapped in the canyon by the temperature inversion.

The 2013 fog event was unusual because of its extent, as shown in the second image, captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. While temperature inversions might happen a couple of times a year, it is rare for so much of the canyon to be enveloped in fog. In the satellite image, taken mid-morning on November 30, the fog seeps into the eastern section of the Grand Canyon, conforming to the shape of the canyon walls. Fog and low clouds hang over the entire landscape east of the canyon.